Due to increasingly improved disability outcomes, and the resultant significantly improved life span, of the multiple sclerosis (MS) population, questions regarding cognitive aging and the prevalence of comorbid Alzheimer disease (AD) have emerged. We describe neuropsychological and MRI-based changes that occurred in an 84-year-old MS patient with comorbid amnestic mild cognitive impairment (a precursor to AD) and cerebrovascular pathology. The neuropsychological examination demonstrated impairment in cognitive processing speed as well as in verbal and visual memory-domains that are potentially affected by any, or all, of the three co-existing diseases. Amyloid-based PET imaging showed increased focal uptake within the gray matter of the occipital lobe. We highlight how these clinical and radiologic observations can inform future research that could elucidate interactions between MS, a probable AD diagnosis, and cerebrovascular pathology in elderly individuals with MS. A comprehensive neuropsychological examination of multiple cognitive domains of individuals with MS may aid in the differential diagnosis of late-in-life cognitive decline.